I see some places people prefer second dining, any reasons why?
A few reasons. Some people just like to eat later. There also tends to be fewer children. The biggest reason is probably so you'll have more time in the ports and on the ship swimming and what not, before you have to get ready for dinner.
Be aware that if you are planning on going to the shows, they'll start close to when first dining would have started so you actually have that much more time to get ready.